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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read.overload6"></a><a class="link" href="overload6.html" title="async_read (6 of 6 overloads)">async_read (6
        of 6 overloads)</a>
</h4></div></div></div>
<p>
          Start an asynchronous operation to read a certain amount of data from a
          stream.
        </p>
<pre class="programlisting">template&lt;
    typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
    typename Allocator,
    typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
    typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a>&gt;
<a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read(
    AsyncReadStream &amp; s,
    basic_streambuf&lt; Allocator &gt; &amp; b,
    CompletionCondition completion_condition,
    ReadHandler &amp;&amp; handler);
</pre>
<p>
          This function is used to asynchronously read a certain number of bytes
          of data from a stream. The function call always returns immediately. The
          asynchronous operation will continue until one of the following conditions
          is true:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
              The supplied buffer is full (that is, it has reached maximum size).
            </li>
<li class="listitem">
              The completion_condition function object returns 0.
            </li>
</ul></div>
<p>
          This operation is implemented in terms of zero or more calls to the stream's
          async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
          The program must ensure that the stream performs no other read operations
          (such as async_read, the stream's async_read_some function, or any other
          composed operations that perform reads) until this operation completes.
        </p>
<h6>
<a name="asio.reference.async_read.overload6.h0"></a>
          <span><a name="asio.reference.async_read.overload6.parameters"></a></span><a class="link" href="overload6.html#asio.reference.async_read.overload6.parameters">Parameters</a>
        </h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">s</span></dt>
<dd><p>
                The stream from which the data is to be read. The type must support
                the AsyncReadStream concept.
              </p></dd>
<dt><span class="term">b</span></dt>
<dd><p>
                A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
                object into which the data will be read. Ownership of the streambuf
                is retained by the caller, which must guarantee that it remains valid
                until the handler is called.
              </p></dd>
<dt><span class="term">completion_condition</span></dt>
<dd>
<p>
                The function object to be called to determine whether the read operation
                is complete. The signature of the function object must be:
</p>
<pre class="programlisting">std::size_t completion_condition(
  // Result of latest async_read_some operation.
  const asio::error_code&amp; error,

  // Number of bytes transferred so far.
  std::size_t bytes_transferred
);
</pre>
<p>
                A return value of 0 indicates that the read operation is complete.
                A non-zero return value indicates the maximum number of bytes to
                be read on the next call to the stream's async_read_some function.
              </p>
</dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
                The handler to be called when the read operation completes. Copies
                will be made of the handler as required. The function signature of
                the handler must be:
</p>
<pre class="programlisting">void handler(
  const asio::error_code&amp; error, // Result of operation.

  std::size_t bytes_transferred           // Number of bytes copied into the
                                          // buffers. If an error occurred,
                                          // this will be the  number of
                                          // bytes successfully transferred
                                          // prior to the error.
);
</pre>
<p>
                Regardless of whether the asynchronous operation completes immediately
                or not, the handler will not be invoked from within this function.
                Invocation of the handler will be performed in a manner equivalent
                to using <code class="computeroutput">asio::io_context::post()</code>.
              </p>
</dd>
</dl>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2018 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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